In the continuing evolution of miniature electrical circuits, a technology named Direct Chip Attachment (DCA) has emerged. DCA refers to directly attaching an integrated circuit (chip) to another circuit such as a printed circuit board. This has the advantage of eliminating the cost of separately packaging the integrated circuit into a carrier before attaching the carrier to a printed circuit board. Another advantage in the elimination of the carrier is the reduction in the surface area necessary to realize an entire circuit. The reduction in surface area is important in the constant push towards ever smaller electronic devices. Such devices include radios, televisions, personal computers, portable telephones, and paging receivers.
In a prior art method of DCA, a printed circuit board (PCB) has metal runners which form conductive paths for circuits on the PCB. The metal runners may typically be made of copper. Pads are formed from the metal runners and are provided for soldering an integrated circuit to the PCB. DCA has been accomplished by depositing a eutectic solder which may consist of a 60% tin and 40% lead on the pads. The general area of the deposited solder is next coated with a liberal amount of solder flux.
Then, an integrated circuit (IC) having bumps is attached to the PCB. The IC has a multiplicity of bumps, the bumps being formed in the IC manufacturing process to provide both electrical and mechanical connection to the PCB. The IC may have anywhere from less than eight to over one hundred and forty bumps depending upon the requirements of the circuitry. The bumps may range in diameter between 2 and 6 mils (0.5 and 1.6 mm) and may be spaced a distance of substantially 5 mils (1.2 mm) or more apart. Each bump is substantially spherical and may consist of a 5% tin and 95% lead compound. This compound gives the bumps a substantially higher melting point than the melting point of the solder used for attaching the IC to the PCB. The pads on the PCB are positioned such that they correspond to the locations of the bumps on the IC.
The IC is then reflow soldered to the PCB by heating the eutectic solder above its melting point. After cooling, both an electrical and a mechanical solder bond is formed between each bump and corresponding pad, thereby attaching the IC to the PCB by melted solder joints.
Because of the very small spacing between the pads, aligning a mask and depositing solder on the pads is an expensive and difficult process. Furthermore, defects in the deposition process may result in either excess or insufficient solder on the PCB which can result in either solder shorts or opens thereby resulting in defective operation and requiring repair of the circuit. For at least these reasons, it is desirable to eliminate the process of depositing eutectic solder on the PCB.
Furthermore, since the alignment of the flux is not critical to the integrity of the solder process, the area of the PCB around the IC is liberally covered with excess flux. This results in residual flux on the PCB. In a subsequent underfilling process, the attached IC and the immediately surrounding PCB area is covered with an epoxy coating which underfills the IC and both protects the IC from the external environment and provides additional mechanical support. If the flux remains on the PCB, the integrity of the underfilling process may be compromised by flux residue at the epoxy to PCB interface. Otherwise, removal of the flux before underfilling necessitates an additional time and expense of a cleaning process. For at least these reasons, it is desirable to improve the underfilling process by eliminating the creation of residual flux.